Various techniques to measure blood flow are known, such as occlusion plethysmography, laser doppler imaging and laser speckle imaging, and thermal dilution measurement. The occlusion plethysmography technique requires restricting venous outflow from a limb, usually the arm. This method of measuring blood flow has disadvantages, such as, occluding blood flow in the entire arm, the blood flow measurements are non-local and slow, and the subject experiences discomfort as a result of the occlusion (e.g., tourniquet).
The laser doppler imaging and laser speckle imaging technique utilizes a laser beam that is incident on the skin. The reflection of the laser beam measures the flow of red blood cells rather than actual blood flow. These two imaging techniques do not provide accurate measurements of blood flow per unit tissue volume and the time for taking the measurements is relatively slow.
The thermal dilution measurement technique requires a heating element in contact with or underneath the skin and a thermocouple or thermistor to measure a temperature change of the skin surface. This technique measures how quickly the skin cools.